Electrolytic production of lead chromates



Patented May 20, 1941 ELECTROLYTIC PRODUCTION OF LEAD CHROMATES ElbertFranklin Weaver, Calumet City, 111., assignor to International smelting& Refining Company, a corporation of Montana No Drawing. ApplicationFebruary 25, 1939, Serial N0. 258,463

4 Claims. (Cl. 204-89) This invention relates to the production of leadchromates and has for its principal object the provision of an improvedelectrolytic method for manufacturing lead chromate products. Thisapplication is a continuation in part of my copending application,Serial No. 134,520, filed April 2, 1937.

In my above-mentioned application I have described an improved processfor the electrolytic manufacture of metal chromate products,particularly lead chro'mates, by means of which it is possible tocontrol the chemical composition and the physical properties of thechromate products formed. The chromate products are formedelectrolytically by passing an electric current from a soluble leadanode through an anolyte comprising a solution of a substance capable ofserving during electrolysis as a solvent for the lead of the anode, adiaphragm, and a catholyte containing chromate ions to a cathode. In myaforementioned application the use of an anolyte comprising an aqueoussolution of sodium acetate or sodium chlorate is referred to. I have nowfound that improved results may be secured in carrying out this processby employing as the anolyte a solution of a substance capable ofyielding nitrate ions during electrolysis. Preferably an aqueoussolution having an alkali metal nitrate dissolved therein is employed asthe anolyte. Sodium nitrate, potassium nitrate, or ammonium nitrate maybe employed as the alkali metal nitrate used in making up the anolyte,but in general the use of sodium nitrate is most economical andconvenient and is preferred.

A complete process embodying the present invention may be carried out ina bifluid electrolytic cell comprising a lead anode immersed in ananolyte containing a substance capable of yielding nitrate ions duringprogress of the electrolytic operation. alkali metal nitrate,advantageously sodium nitrate, preferably is employed as the anolyte.The anolyte is separated from a catholyte in the cell by a permeablediaphragm. The catholyte comprises a solution containing a substancecapable of yielding chromate ions during the course of the electrolyticoperation. An aqueous solu- An aqueous solution of anv tion containingsodium chromate or sodium dichromate is in general preferred for use asthe catholyte. electric current is immersed in the catholyte.

The complete apparatus employed preferably includes means forcirculating anolyte and catholyte solution through the anolyte andcatholyte compartments in the cell, and to and from ano- A cathodecapable of conducting.

lyte and catholyte storage compartments out. side the cell. Means areincluded in the anolyte circulation system outside the cell forseparating the chromate product formedin the anolyte. Means are providedfor adding a substance capable of yielding chromate ions to thecatholyte,-

. products produced by heretofore known methods.

For example, lead chromate pigments produced electrolytically with theuse of a nitrate anolyte generally are of finer and more uniformparticle size, exhibit a greater degree of color or tint retention, aremore resistant to color changes induced byexposure of the pigment tolight, and give cleaner and clearer tints and are less subject tochalklness when employed as pigments in paint or similar products.

In addition to yielding a superior product, the improved method of theinvention results in substantial operating economies. Forexample, aprocess employing an acetate anolyte operates, at atemperature of about25 C. with a cell voltage of about 4.3, whereas the new processinvolving the use of a nitrate anolyte operates at the same temperaturewith'a cell voltage of about 3.5,. other factors remaining the same. Inthe case of a cell unit operating with a total current consumption of4,000 amperes (a typical current consumption for a commercial cellutilized in the electrolytic production of lead chromate), this reducedvoltage results in a power saving of about 3,200 watts. Even inoperating at electrically more emcient higher temperatures, the nitrateprocess of the invention leads to substantial power savings. At about 500., for example, the cell voltage will amount to about 2.6 volts whenusing an acetate anolyte, but will amount only to about 2.3 volts whenemploying a nitrate anolyte, other factors remaining the same. Thisreduced voltage results in a, power saving of about 1,200 watts per celloperating at 4,000 amperes. 1

In addition to the foregoing advantages, the

use of nitrate anolytes leads to further economies due to the fact thatnitrate salts are less expensive and more readily procured and handledthan the corresponding acetate or chlorate salts, which heretofore havebeen employed in electrolytic manufacture of lead chromates.

In carrying out a process for the production of lead chromate productsin which the improvements of the present invention are embodied. theprinciples and procedures of operation set forth in my above-mentionedcopending application advantageously are followed in order to insurecontrolled production of chromate products having predetermined chemicalcomposition and physical properties, except, of course, that a nitrateanolyte is employed in place of the acetate or chlorate anolyte referredto in the said application. The nitrate anolyte preferably is an aqueoussolution containing between 2 and 8% by weight of sodium nitrate orother alkali metal (including ammonium) nitrate. The nitrateconcentration preferably is not less than about 2% by weight of theanolytenfor at lower concentrations the cell voltage increases touneconomic values. while a concentration of nitrate in excess of about8% by weight of the anolyte leads to uneconomic nitrate consumptionwithout beneficially lowering the cell voltage. An anolyte containingabout 4% by weight of sodium nitrate or other alkali metal nitrate isparticularly satisfactory.

Depending on the type of chromate product desired to be produced, theanolyte also contains small percentages of hydroxyl ions together withchromate ions (when a basic chromate product is sought) or chromate ionsand dichromate ions, or only dichromate ions (when normal chromateproducts are desired). A typical analysis of an anolyte employed inpreparing a basic lead chromate product may show the followingingredients: Per cent by weight Sodium nitrate 4.0 Sodium hydroxide 0.04to 0.06 Sodium chromate 0.02 to 0.10

As pointed out in my aforementioned application, proper control of thecell product is achieved by controlling the concentrations of hydroxylions, chromate ions, and dichromate ions in the electrdiytes,particularly in the anolyte. The pH of the anolyte provides a convenientindication of ese ion concentrations; and depending upon the pH of theanolyte different products may be formed, substantially as indicated inthe following tabulation:

Anolyte pH Ions present Product Color of product 7.5 to 9.5 HydroxylBasic lead Orange to red orange.

and chrm chromate. ma 6.0 to 7.5 Chromate Normallead Medium yellow tomeand dichromate. dium orange. chromate.-

Below 6.0... Dichromate. do.' Yellow to pale yellow.

age of sodium hydroxide (when preparing basic chromates) or togetherwith sodium dichromate (when preparing normal chromate products).

When preparing certain normal chromate products, the catholyte may befree of chromate ions and contain only a suitable percentage ofdichromate ions. (In the claims, the term "chromate ions". is used inreferring to solutions containing either chromate or dichromate ions.)

The product of a process embodying thepresent invention is influenced bythe temperature at which the process is carried out. For example,

when producing basic lead chromates, it has been found desirable tomaintain the temperature of the cell above about C. In producing thenormal chromates, a temperature of 25 C. or less is preferable. Anelevated temperature results in formation of a product of deeper (moreorange or red) colorthan is produced at ordinary temperatures.

The current density advantageously is maintained at about 15 to 30amperes per square foot of-cathode surface, and the cell voltageadvantageously is maintained at about 3.2 to 3.? volts and operating atabout 25 C. and between about 2.1 to 2.5 volts when operating at atemperature of about 50 C.

During the course of the electrolysis, the sodium nitrate present in theanolyte reacts with the lead of the anode to form lead nitrate andliberate sodium ions. The sodium ions migrate to the cathode where theycombine with water to form sodium hydroxide and liberate hydrogen.

Simultaneously an equivalent amount of chromate ions are liberated atthe cathode and migrate toward the anode. The chromate ions migratinginto the anolyte react with the lead nitrate formed therein to producelead chromate and to regenerate sodium nitrate. The increase in freehydroxyl ions in the catholyte resulting from these reactions iscompensated for by the addition to the catholyte of .chromic acid orchromic anhydride. The following equations are illustrative of thechemical reactions taking place in the production of a normal. leadchromate product.

The only' reagents actually, consumed in the process are lead from theanode and chromic acid (or anhydride) which is added to the catho-.lyte. The sodium nitrate is wholly regenerated, .but minor lossesthereof due to spillage, leakage and other mechanical causes are made upby periodically adding small amounts 'of sodium nitrate to the anolyte.

I claim:

1. In a method for producing lead chromates electrolytically involvingpassing an electric our- 2,242,634 ing chromat'e ions to acathode, theimprovement which comprises employing as the anolyte an aqueous solutionhaving dissolved therein-about 4% by weight of an alkali metal nitrateand furtheremploying 'a cathode current density of about 15 to 30amperes per square foot.

3. In a method for producing lead chromates electrolytically involvingpassing an eleotric current from a soluble lead anode through ananolyte, a diaphragm, and a. catholyte containing-chromate ions to acathode, the improvement whichcomprises employing as the anolyte anaqueous solution having dissolved therein about 2 to 8% by weight ofsodium nitrate and, further employing a cathode currentv density ofabout 15 to 30 amperes per square foot.

4. In a method for producing'lead chromates electrolytically involvingpassing an electric current from a soluble lead anode throughan-anolyte, adiaphragm, and a. catholyte containing chromate ions to acathode, the improvement whichcomprises employing-as the anolyte anaqueous solution having dissolved therein about 4% by weight of sodiumnitrate and further employing a cathode current density ofabout 15 to 30amperes per square foot.

'ELBERT mm WEAVER.

